I suppose I should start this by saying that I know next to nothing about guns, but a question occurred to me the other day and a Google search didn't help me find the answer, so I joined these forums to ask you folks.

From what I understand, a bullet can be likened to a bottle full of gunpowder, where the actual projectile is the cork. If this is indeed the case, why does the size of the "cork" vary with different kinds (caliber, right?) of bullets? Is it a tradition thing, where different bullet sizes are made because they've always been made? It seems, to my uneducated mind, that it would be cheaper and more efficient to use the same cork (or at least the same diameter) for every bottle and vary instead the size of the bottle's neck.

In any case, thank you in advance for your help. If my question is incredibly stupid, I assure you it was without my knowledge.

A short answer would be "we already do that". There are lots of cartridges (the primer, case, gunpowder and bullet together) that do essentially that - same bullet size in diameter and weight - but vastly different case(bottle) sizes and powder charges. Two extremes of this example could be .223 and .22-250. Massively different case sizes and powder loads, massively different performance out of the barrel with essentially the same bullet (cork).

One of the big things to consider is, one size(diameter/weight) of bullet isn't ideal for all situations. a small bullet going REALLY fast doesn't do what needs done to take down large game - think African Cape Buffalo. They require a much, much bigger (diameter/weight) bullet with a much different case to handle it and the pressures needed to get that size of bullet moving properly down the barrel so the bullet can do it's job.

There are myriad answers to your question, but these 2 will start things for you. There's also the notion of personal preference and the need to "mess with things" in there too, but I'll leave things be. For now.

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"It ain't braggin' if you CAN DO IT!" - Elmer Keith

I'm going with money, its all about money- make your own rifle and caliber and you just made yourself some business as long as your guns sell and let's not forget some people could only afford a cheap caliber where a other folks can afford larger calibers back in the day. Other thing is tradition shotguns are traditionally used for small game and birds and rifles are for larger game, you dont wanna use a .22 on a elk or a 50bmg on a groundhog. Then you have the notion of war calibers used to maim but not kill your enemy~5.56~ its a crazy ammo world. I love to see bullet collections.

You can go down a dark and winding road quite easily.
It's all about physics.

Heavier bullets hold their velocity better than light bullets. Shooting long range? Better get a heavy.

Heavier bullets are harder to stop than light bullets. Heavy bullets mean deep penetration for hunters. Middleweights offer moderate penetration for self defense. Zippy lightweights make for poor penetration but work great for paper targets.

You can only get so heavy before things get out of control. I don't think a 22 grain bullet tipping the scales at 190 grains would be very easy to use. However, it's a great weight to use for long range 30 caliber targets. It is a fine weight for self defense too but you want to spread out the surface area so it does more damage to the bad guy instead of zipping right through like an ice pick.

Old ammo designs like the 45-70, 44-40, and 45 Colt have big bullets and fat cases because the metals of the day would not hold heavy pressure without getting huge and bulky. To put down a critter you needed a heavy slug of lead doing significant damage. That meant a big fat frontal area. They had to use a bunch of black powder to do what we manage today with small amounts of smokeless powder.
As the years go by, metals improve, powders develop, and performance goes up.

Each cartridge developed for it's own reason. They're kinda like designer clothes. What appeals to one may not appeal to another.

You got pretty close when you suggested the bullet was a cork. That is EXACTLY what it does. It plugs the only avenue of escape for a bunch of burning powder. In fact, how tightly a bullet is stuffed in a case changes the characteristics of the powder burn. Some powders need help building pressure and putting a heavy crimp on a bullet helps do just that.

The powder provides the power but not like many would think.
Most people (including lots of shooters) don't realize is that modern smokeless powder doesn't go *BOOM* like the cartoons unless the pressure is contained. I demonstrated this to friends many years ago but pouring a bit of smokeless powder in a piece of paper and folding it up. When lit, it burned like, well, a piece of paper. The smoke was smellier and the flames had a bit of a different color but there was no crazy burn. If that same charge had been restrained by the chamber in a gun and the bullet acting like a cork things would have been WAY different.

Fireworks operate on the same principle. If you were to cut open one of your Black Cat poppers and dump the powder out it would not pop when lit. It would burn like fizzly paper. That paper casing holds back the burn. Pressure builds and it goes boom. Now, this happens VERY rapidly but that's how it works.

If you need a bullet to go fast the pressure has to be big. There is a limit on how much pressure a gun can hold. If you've topped it out and still want more the answer is a bigger cartridge case. You still have the same limit but you can use a powder that burns slower. It will make the same peak pressure but it takes longer to do it. This means it pushes hard for a longer period of time. More gas is shoving the bullet down the tube.

Dig back into your physics class and pull out that phrase area under the curve. Yes, an old bell curve. That's what the cartridge designers are dealing with.

You also get into getting as much a possible from as little gun as possible and things really get twisted around.

I think that's a wee bit more than you asked but maybe it'll help understand what is going on.

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